I have 3 out of 4 secrets and the 4th appears nowhere when I search for the 4th secret by enabling "Show Secrets always" and IDDTing.

I thought it might be not visible, so I fired up Doombuilder and search for all secrets. But there are still three! How is this possible? Does it happen to other WADs, is that a bug that can happen depending on how you make your maps?